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Leak Testing Basics
Basic Principles
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leak testing - basic principles
What is a Leak?
A leak is a flow of gas (or liquid) through the wall of a vessel
(via an imperfection
such as a hole, crack or bad seal).
Leaks require a pressure difference to generate the flow; they
always go from
higher pressure to lower pressure.
Leaks are pictured as going from positive pressure (inside an
object) to outside (atatmospheric pressure).
This is not always the case (a leak could be from atmosphere to
inside anevacuated object), but it helps to think about it this way
because the units and
terminology are based on this model.
Leak Testing - what units do you use?
For leaks of air into atmosphere, units are usually expressed as
mm3 or cm3 (cc)
per second or minute. So 16.6 mm3/sec = 1 cm3/min. A bubble
under water is
about 30 - 50 mm3, so 1 bubble per second is about 30 mm3/sec or
2 cm3/min. A
standard unit of leakage which takes account of air pressure is
the mbarl/sec.(Millibar-litre per second). A leak into atmosphere
of 1 mbarl/sec is equivalent to
a volume leak of 1000 mm3/sec.
Leak Testing - what technique should be used?
Key questions at the start of any leak test requirement
are:-
What size is the component and what isits internal volume?
Is there access to inside or is it a sealedunit?
What is the leak limit? Is it rigid or flexible?
Does it have hidden internal volumes
that may affect leak measurements?Are parts at ambient
temperature?
Are the parts clean and dry?What is the surface finish of any
sealing
surfaces?
Based on the acceptable leak rate limit (shown in ml/sec) alone
the following test
method can be used
Tracer Gas Flow Rate
Water Submersal / Dunk Tank
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Leak Testing - generic systems
pressure/vacuum
The test piece and the reference volume
are simultaneously pressurised (or
evacuated) to a preset pressure. The air
in the system is then allowed to stabilise,with the supply
valves all closed. The
Differential Pressure Transducer is
automatically zeroed.
After this stabilisation time, the pressure
change in the test piece is compared tothe pressure change in
the reference
volume, using the Transducer. If the testpiece is leaking, the
difference willincrease and be measured, an alarm limit
may be set for a pass/fail decision.
The sequence is fully automatic, the
accuracy and sensitivity of the system isdefined by the method
of setting the
preset pressure together with the qualityand type of control
valves andDifferential Pressure Transducer.
helium systems
A vacuum pump evacuates the testchamber and test piece
simultaneously
to a preset vacuum. At this preset level,the chamber and the
test piece areisolated and the chamber evacuated
further to a very low pressure. Apositive pressure variation is
therefore
created between the test piece and thechamber.
Helium gas is then introduced into thetest piece, often in a 10%
concentration.A Mass Spectrometer analyses a
sample from the chamber as the vacuumcontinues to be drawn. The
MassSpectrometer measures the helium
leakage and sets the pass/fail decision.
The test piece pressure is often
compared to the chamber pressurebefore dosing with helium, to
avoid
saturating the Mass Spectrometer in the
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